1. Field of the Invention
This invention relates generally to transition metal oxide semiconductors, and, more specifically to transition metal oxide compositions of matter and methods for insertion of a halogen moiety such as fluorine into said metal oxides to provide a chemically stable, highly conductive semi-conductive material, which has particular utility for catalytic reactions, semiconductor devices, and transparent flat screen displays.
2. Brief Description of the Related Art
The application of transition metal oxide (TMO) semiconductors for charge-based devices is becoming prevalent for chemical, electrical, and energy applications. TMO semiconductors (e.g. TiO2, CO2O3, CeO2, ZnO, SnO, and WO2) have been widely studied for use in such applications as photo catalysts, photovoltaic solar cells, transparent oxide thin film transistors, resistive switching memory and chemical sensors. Among the TMOs, titanium oxide is among one of the most widely studied for use in photo catalysis, light harvesting, resistive switching memory, and chemical sensing as an intrinsic n-type semiconductor.
In TMO semiconductors, oxygen (O) vacancies act as intrinsic n-type donors leading to enhanced surface conductivity compared to the fully oxidized stoichiometric metal oxide. Consequently control of TMO surface conductivity is possible by controlling O vacancy concentration. However, the intrinsic doping of TMOs through manipulation of O vacancies is equivalent to the formation of reduced oxides, which eventually results in metallic conduction. This not only compromises the use of the metal oxides as a semiconductor material, but also gives rise to charge trapping at defect (i.e. O vacancy) locations, and surface Fermi energy (Ef) level pinning. Additionally, the electric activity (e.g. charge trapping) and chemical reactivity of O vacancies create problems of material stability/reliability. In fact, these materials are so unstable that in liquid crystal display applications, they will not work after a period of just several hours. Consequently, highly conductive titanium oxide is seldom useful for device architecture, and fabrication of highly conductive, defect-free titanium oxide remains an important challenge.
The extrinsic chemical doping of stoichiometric titanium oxide using impurity dopants (e.g. N, S, and C) by colloidal synthesis methods to modify the electronic structure and achieve visible light absorption has been actively studied for photo-catalytic applications. Additionally, metal impurity (e.g. V, Ni, and Nb) doping of titanium oxide has been used to modify EF and increase conductivity. However, in each of these cases, the semiconductor properties of titanium oxide have been found to be compromised by the formation of metallic mid-gap states having an energy level less than intrinsic bandgap energy.